Since World War II, the U.S. Army has used approximately 20 times more energy per soldier while reducing the number of soldiers deployed. This undoubtedly will continue in the future as new capabilities with higher-power weapons, novel sensors and robots, and advanced computing are envisioned. This highlights the importance of energy supply and management for the future battlefield.

When considering Army maneuver, one key objective is minimizing the number of trucks carrying materials to the battlefield. As Army Futures Command explores future autonomous vehicles and active protective systems, the concern becomes less about lost lives (which was a major factor in Iraq and Afghanistan) and more about lost transport vehicles, fuel, water, munitions and other supplies, as autonomous systems would not require the physical presence of a human.

Looking at the transportation fuels used today, diesel, JP8 and biodiesel clearly have the highest volumetric energy density. And this is the most important measurement, as supply trucks generally “cube out” before they “weigh out.”

The importance of this is readily apparent when you consider that transporting an equivalent amount of energy using cooled liquefied or compressed hydrogen would require four to seven times as many supply trucks compared to the current fuel of choice, JP8. Other possible fuel choices are shown in the accompanying chart.

Hybrid propulsion (i.e., an internal combustion engine coupled to battery storage, power electronics and an electric drive system) is close to ideal for ground combat vehicles, providing improved fuel efficiency, extended range, faster acceleration, exportable power, silent watch and quick refueling – albeit with limited-range silent mobility. No wonder the Army has launched a number of studies on this technology, some internal; one as part of a $32 million project with a British defense company; and one jointly with Japan. Interestingly, Israel also is considering the use of hybrid propulsion for its Merkava main battle tank.

The performance improvement opportunities made possible are significant. Brake thermal efficiency, or BTE, improvements in compression ignition engines, spurred on in large part by the Energy Department’s SuperTruck projects, have been impressive, increasing from around 40 percent to more than 50 percent.

So what about the advanced nuclear power plants that are currently under development? Will they enable the pure-battery electric ground combat vehicles that some military experts seek?

Project Pele (1 to 5 megawatt electrical units) – a mobile nuclear power plant, or MNPP, under development for operating bases – doesn’t meet the need for a ground combat vehicle fleet. These 39-ton mobile nuclear reactors are being designed to operate within three days of delivery and to be safely removed in as few as seven days.

While such a nuclear plant may prove to be attractive for 24/7 power at long-term military facilities that require substantial energy for sustainment operations, it falls far short of what is required to power an expeditionary force.

Stepping up a bit, NuScale’s smallest modular reactor (77 MWe) might be adequate for a reasonable number of the lightest vehicles, but its installation is lengthy and its footprint large, essentially mandating it to be a permanent installation.

In our National Academies of Sciences, Engineering, and Medicine study, “Powering the U.S. Army of the Future,” we note that an all-electric main battle tank will require between 3.5 and 7 megawatt hours of energy for a full recharge.

Similarly, we estimated that all-electric versions of a Joint Light Tactical Vehicle and an Infantry Squad Vehicle – much lighter vehicles – will require roughly 0.65 MWh and 0.29 MWh, respectively.

Turning this into the number of mobile nuclear power plants (MNPP) or small modular reactors (SMR) that would be required for recharging any number of vehicles is simple math.

As an example, to recharge 15 JLTVs within an hour, assuming Project Pele achieves it aspirational goal of 5 MWe, roughly two MNPPs would be required. The number of required MNPPs grows to 10 if each MNPP ends up being capable of only producing 1 MWe.

We argue that these energy and power requirements need to be vetted against the technical realities of the energy supply with coordination between energy experts, vehicle engineers and war fighters.

We stress that this is not a battery issue. It can’t be solved with advancements in battery energy density. It is simply a matter of finding sufficient electrical power in the battlefield for recharging. And it argues against any high-volume application of BEV military vehicles.

The case for continued use of internal combustion engines in the supply trucks within a convoy is even more compelling. A number of battery electric trucks are under development, and they certainly have their place in the commercial realm in cities where pollution is an issue. And they can return at night to a central depot for recharging.

However, most of these have a range of 250 miles, with one claiming 500 miles, albeit with a payload reduction. Compare this to a conventionally powered Class 8 truck (18 wheeler), which, with its two 150-gallon fuel tanks, can travel roughly 2,100 miles without refueling – or almost 4,000 miles without refueling if leveraging the latest SuperTruck technologies.

This does not mandate that the Army continue to be a “CO2 polluter” for the foreseeable future. The term “liquid hydrocarbon fuel” is not synonymous with “liquid fossil fuel.” Many fleets today are using pure B100 biodiesel certified to BQ-9000 or EN 14214 standards. Renewable diesel and synthetic aviation fuels are under development and will be needed for other high-weight applications, such as commercial aircraft. Technologies, such as closed-loop combustion that is already widely available in the commercial market, can enable seamless switching between these fuel alternatives.

The bottom line: Electrification of ground combat vehicles is highly desirable, but it should take the form of hybrid electric vehicles with internal combustion engines. Key to their successful implementation is making sure that investment and development are focused on the correct technologies.

Futures Command has made positive and early strides to pursue electrification of ground combat vehicles. However, until an infrastructure is in place that can power such vehicles in remote locations, AFC leadership would be wise to deploy hybrid electric vehicles for maximum battlefield versatility.

John Koszewnik and John Luginsland co-chair the National Academies of Sciences, Engineering, and Medicine study “Powering the US Army of the Future.” Koszewnik is a former chief technical officer of Achates Power. Luginsland is a senior scientist for Confluent Sciences as well as a fellow at the Institute of Electrical and Electronics Engineers and the Air Force Research Laboratory.